Optical quantification of harmonic acoustic radiation force excitation in a tissue-mimicking phantom

Optical tracking was used to characterize acoustic radiation force (ARF) induced displacements in a tissue-mimicking phantom. Amplitude modulated (AM) 3.3 MHz ultrasound was used to induce ARF in the phantom which was embedded with 10 {\mu}m microspheres that were tracked using a microscope objective and high speed camera. For sine and square AM the harmonic components of the fundamental and second and third harmonic frequencies were measured. The displacement amplitudes were found to increase linearly with ARF up to 10 {\mu}m with sine modulation having 19.5% lower peak-to-peak amplitude values than square modulation. Square modulation produced almost no second harmonic but energy was present in the third harmonic. For the sine modulation energy was present in the second harmonic and low energy in the third harmonic. A finite element model was used to simulate the deformation and was both qualitatively and quantitatively in agreement with the measurements.